Photochromic or phototropic glasses, as such have been variously termed, had their genesis in U.S. Pat. No. 3,208,860. That patent discloses the mechanism of photochromism as being dependent upon the presence of silver halide crystals in a silicate-based glass, i.e., crystals of silver chloride, silver bromide, and/or silver iodide in a matrix of a silicate glass. The patent teaches the preferred matrix glasses to have compositions within the alkali metal aluminoborosilicate system, viz, glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and at least one alkali metal oxide selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O. Photochromic glasses having base compositions within the alkali metal aluminoborosilicate field have constituted by far the greatest number of products marketed commercially.
Although numerous other base glass compositions have been disclosed as suitable matrices for silver halide crystals to thereby produce articles exhibiting photochromic behavior, only those glasses encompassed within the alkali metal aluminophosphate system have enjoyed any commercial success. U.S. Pat. No. 3,876,436 broadly disclose such glasses and delineates preferred ranges thereof as consisting essentially, expressed in terms of weight percent on the oxide basis, of 60-80% Al.sub.2 O.sub.3 +SiO.sub.2 +P.sub.2 O.sub.5, consisting of 20-34% Al.sub.2 O.sub.3, 0-40% SiO.sub.2, and 17-48% P.sub.2 O.sub.5, wherein P.sub.2 O.sub.5 :Al.sub.2 O.sub.3 is not less than 0.7, 0-19% B.sub.2 O.sub.3, and at least 10% Li.sub.2 O and/or Na.sub.2 /O and/or K.sub.2 O.
Many subsequent patents have been granted involving relatively narrow ranges of base compositions within both the alkali metal aluminoborosilicate field and the alkali metal aluminophosphate system. These later patents cover compositions wherein the photochromic properties are improved and/or other physical and chemical characteristics of the glass are beneficially modified.
U.S. Pat. No. 4,190,451 is illustrative of the former field, disclosing glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 8-20% Li.sub.2 O+Na.sub.2 O+K.sub.2 O+Cs.sub.2 O consisting of 0-2.5% Li.sub.2 O, 0-9% Na.sub.2 O, 0-17% K.sub.2 O, and 0-6% Cs.sub.2 O, 5-25% Al.sub.2 O.sub.3, 14-23% B.sub.2 O.sub.3, 0-25% P.sub.2 O.sub.5, and 20-65% SiO.sub.2. U.S. Pat. No. 4,092,174 is an example of the latter system, describing glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 7-20.5% Li.sub.2 O+Na.sub.2 O+K.sub.2 O, wherein the maximum content of Li.sub.2 O is 5%, 13-36.5% Al.sub.2 O.sub.3, 7-28% B.sub.2 O.sub.3, 8.5-25% SiO.sub.2, and 7.5-33.5% P.sub.2 O.sub.5.
Experience has indicated that copper behaves as a sensitizing agent such that its presence is demanded to insure good photochromic properties in a silver-containing glass, i.e., the glass, when subjected to actinic radiation, customarily ultraviolet radiation, will quickly darken to a low luminous transmittance and, when removed from exposure to actinic radiation, will rapidly fade back toward its original transmittance. As utilized here, the expression luminous transmittance of a glass is defined as the value Y delineated in terms of the 1931 C.I.E. trichromatic colorimeter system employing the light source Illuminant C. This colorimetric system and light source are described by A. C. Hardy in the Handbook of Colorimetry, Technology Press, M.I.T., Cambridge, Mass. (1936). Experience has also demonstrated that a combination of silver chloride and silver bromide crystals normally imparts the most ideal photochromic character to the glass. Therefore, the workers in the field of silver halide-containing photochromic glass have defined the necessary "photochromic elements" as silver, chloride, bromide, and copper expressed as CuO.
Unfortunately, the chemical durability of alkali metal aluminophosphate glasses is not as great as that of glasses in the alkali metal aluminoborosilicate field. This deficiency has been especially vexing in the finishing and cleaning of ophthalmic lenses by lens manufacturers. Moreover, the mechanical strength of the former glasses, even after chemical strengthening, is less than that of the silicate glasses. Those two factors have been mainly responsible for the greater commercial use of alkali metal aluminoborosilicate base compositions as matrices for silver halide crystal-containing photochromic glasses.
As is explained in U.S. Pat. No. 4,190,451, the content of CuO in the glass is critical to not only induce photochromic behavior therein, but also to impart relative temperature independence to the glass, i.e., the photochromic character of the glass does not change drastically with variations in ambient temperature. Thus, excessive amounts of CuO cause a reduction in the darkening capability of a glass at room temperature and higher, and also effect increased darkening when exposed to actinic radiation at low temperatures. Accordingly, for each glass composition, a delicate balance in CuO content is required to be struck to achieve desirable photochromic properties. Maintaining this balance is a very difficult problem from a practical point of view since the amount of CuO used is so small, a minimum of about 0.005%, but generally less than 0.03%, and commonly less than 0.015%. Greater quantities adversely affect the overall photochromic properties plus impart a permanent coloration in the glass.
Electron paramagnetic resonance (EPR) studies have indicated that it is the Cu.sup.+ ion that performs as a sensitizer to thereby confer photochromic behavior to the glass. Thus, the photochromic mechanism operating in alkali metal aluminoborosilicate base glasses is dependent upon moving the Cu.sup.+ -Cu.sup.+2 equilibrium toward Cu.sup.+ to a sufficient extent that there are adequate Cu.sup.+ ions available to cause glass darkening. EPR examinations have demonstrated that more than one-half and, preferably, at least two-thirds of the copper will be available in the Cu.sup.+ state. This circumstance would augur the use of a reducing agent in the glass.
U.S. Pat. No. 3,208,860 suggests the use of a reducing agent in an amount sufficient to convert silver ions to metallic silver atoms. Unfortunately, however, extensive reduction leads to the generation of a permanent amber coloration in the glass as a result of the presence of colloidal silver particles therein. Therefore, the primary objective of the subject invention is to insure that more than one-half of the copper is present in the Cu.sup.+ state in a silver halide-containing, alkali metal aluminoborosilicate glass to achieve good photochromic properties therein, but excessive amounts of colloidal silver are not present.
The oxidation state of the copper is important in permitting adjustments to be made in other components of the glass and in glass melting and forming practices. For example, higher alkali metal contents can be tolerated in the glass composition, that feature frequently increasing the fading rate of the glass, while maintaining the darkening capability thereof. Further, the substantial presence of Cu.sup.+ ions enables the glass to be melted at lower temperatures with no adverse effect upon the photochromic behavior exhibited thereby. The concern regarding the oxidation state of the copper becomes especially critical, however, when highly oxidizing materials, such as the colorants Cr.sub.2 O.sub.3, MnO.sub.2, CeO.sub.2, and V.sub.2 O.sub.5, are incorporated into the composition. For example, whereas the Cr.sup.+3 ion is well-recognized as producing a deep, high purity green coloration to a glass, its utility in the commercially-marketed alkali metal aluminosilicate photochromic glasses has been limited because it adversely affects the photochromic mechanism. EPR studies indicated that, as chromium is added to the commercial glass, the equilibrium Cu.sup.+ -Cu.sup.+2 is shifted significantly toward Cu.sup.+2. This shift dramatically decreases the photochromic behavior of the glass. Accordingly, a second major objective of the instant invention is to provide means for coping with the inclusion of highly oxidizing ions in the glass while maintaining desirable photochromic performance.